Hydrocarbons (such as oil, condensate, and gas) may be produced from wells that are drilled into formations containing them. For a variety of reasons, such as low permeability of the reservoirs or damage to the formation caused by drilling and completion of the well, or other reasons resulting in low conductivity of the hydrocarbons to the well, the flow of hydrocarbons into the well may be undesirably low. In this case, the well is “stimulated,” for example, using hydraulic fracturing, chemical (such as an acid) stimulation, or a combination of the two (often referred to as acid fracturing or fracture acidizing).
Hydraulic and acid fracturing treatments may include two stages. A first stage comprises pumping a viscous fluid, called a pad, that is typically free of proppants, into the formation at a rate and pressure high enough to break down the formation to create fracture(s) therein. In a subsequent second stage, a proppant-laden slurry is pumped into the formation in order to transport proppant into the fracture(s) created in the first stage. In “acid” fracturing, the second stage fluid may contain an acid or other chemical, such as a chelating agent, that can assist in dissolving part of the rock, causing irregular etching of the fracture face and removal of some of the mineral matter, which results in the fracture not completely closing when the pumping is stopped. Occasionally, hydraulic fracturing may be done without a highly viscosified fluid (such as water) to minimize the damage caused by polymers or the cost of other viscosifiers. After finishing pumping, the fracture closes onto the proppant, which keeps the fracture open for the formation fluid (e.g., hydrocarbons) to flow to the wellbore of the well. The performance characteristics of the proppant contribute to the overall effectiveness of the fracturing stimulation.
Proppant is typically made of materials such as sand, glass beads, ceramic beads, or other materials. Sand is used frequently as the proppant for fracture treatments. Sand and ceramic proppant with close-to-spherical shapes are the most frequently used type of proppant in hydraulic fracturing. However, there has also been interest in employing non-spherical proppants. A mechanically connected proppant pack composed of non-spherical particles may exhibit a higher porosity compared to a pack of spheres. Furthermore, non-spherical particles may exhibit different settling properties compared to spherical particles, as well as reduce embedment due to a larger contact area from proppant to fracture face. Another perceived benefit of non-spherical proppants is the enhanced mechanical interactions with fibers for proppant transport (U.S. Pat. No. 8,230,925) in fracturing services in which the fracture treatment fluid contains fibers in addition to proppant. Also, the mechanically connected proppant pack composed of non-spherical particles may prevent flow back of the proppant.
Proppant morphology is a feature that is often exploited as a differentiator in proppant performance. Regular sand is typically not perfectly smooth and the degree of sphericity varies between grains. Some ceramic proppants have been developed to achieve high degrees of smoothness and sphericity. See, for example, U.S. Pat. No. 8,883,693. Patents directed to proppants of various compositions with irregular shapes or various sizes include U.S. Pat. No. 6,725,930 (metallic wire), U.S. Pat. No. 7,806,181 (polymer particles), U.S. Pat. Nos. 7,849,923 and 7,931,966, and U.S. Patent Application Publication No. 2011/0180259 (plates) and U.S. Patent Application Publication Nos. 2008/0234146 and 2012/0247764 (non-spherical mixed with spherical). Yinghui Liu et al. published an article on an X-shaped high-drag proppant to minimize settling. See Liu et al., “A New Generation High-Drag Proppant: Prototype Development, Laboratory Testing, and Hydraulic Fracturing Modeling,” SPE-173338, Society of Petroleum Engineers, February, 2015.
The so-called drip-casting manufacturing technique has been adapted for the manufacture of spherical ceramic proppants. Drip-casting substitutes conventional ways of pelletizing (also called granulating) ceramic proppant such as using high intensity mixers and pan granulators. Vibration-induced dripping (or drip-casting) was first developed to produce nuclear fuel pellets. See U.S. Pat. No. 4,060,497. It has subsequently evolved into applications for metal and ceramic microspheres for grinding media, pharmaceuticals and food industry. An application of vibration-induced dripping to aluminum oxide spheres is described in U.S. Pat. No. 5,500,162. The production of the microspheres is achieved through vibration-provoked dripping of a chemical solution through a nozzle. The falling drops are surrounded by a reaction gas, which causes the droplet to gel prior to entering the reaction liquid (to further gel). Using a similar approach, U.S. Pat. No. 6,197,073 covers the production of aluminum oxide beads by flowing a sol or suspension of aluminum oxide through a vibrating nozzle plate to form droplets that are pre-solidified with gaseous ammonia before their drop into ammonia solution. U.S. Patent Application Publication No. 2006/0016598 describes the drip-casting to manufacture a high-strength, light-weight ceramic proppant. U.S. Pat. No. 8,883,693 describes the application of the drip-casting process to make ceramic proppant.
What is still desired are new non-spherical particles and novel methods of making such non-spherical particles.